Interpretive Summary: This book chapter discusses photosynthesis and stomatal conductance of plants grown under elevated [CO2] in manage ecosystems. Elevated atmospheric [CO2] increases leaf carbon uptake (photosynthesis) and decreases leaf water loss (stomatal conductance). Under completely open-air field conditions, photosynthesis is increased less and stomatal conductance decreased more than expected from controlled environment studies. Photosynthesis is also stimulated less at the beginning and end of the day, when light levels are low. The mechanisms behind these responses are discussed in this chapter.

Technical Abstract:
Free Air Concentration Enrichment experiments have provided the only fully open-air treatment of crops with the elevations of [CO2] anticipated to occur by the middle of this century. Observed effects differ substantially from prior observations in chambers and from theoretical expectations.
The increase in light saturated photosynthesis is substantially less than observed in chambers or expected from theory. This is particularly marked in annual C3 grain crops, where the mean increase of 18% was just half the theoretical expectation.
For annual crops over the growing season and for perennial herbage crops grown for 10 years, there is no evidence of any long term loss of stimulation of photosynthesis by elevated [CO2] at either high or low N supplies.
Consistent with theoretical expectation, increase in photosynthesis is pronounced during periods of light saturation around mid-day, but minimal under the light-limiting conditions following dawn and preceding dusk. Decrease in stomatal conductance is almost double that observed in chamber experiments.
In line with theoretical expectation on changes that would increase resource use efficiency in elevated [CO2] but in contrast to chamber studies, a significant selective loss of in vivo Rubisco activity and content has been observed. While smaller than the loss in Rubisco activity, a decrease in capacity for regeneration of RubP is also observed and this explains most of the short-fall in observed versus theoretical increase in leaf photosynthesis.